Contactless smartcards receive both electrical power and data from a modulated high frequency electromagnetic signal emitted by a card reader via an inductively coupled coil on the card. The electromagnetic field strength of the signal and hence the voltage and current generated within the smartcard, depends on the distance of the smartcard from the reader. Therefore, if the coil and associated circuit is designed to adequately energize the card at a specified maximum working distance, it will generate much higher voltage and current as the smartcard is moved closer to the reader that serves as the signal source.
A voltage regulator is used to protect the electronic circuits in such smartcards from being damaged by excessive voltage. Shunt voltage regulators maintain a steady voltage output by sinking excess current from the input. In the shunt arrangement, no part of electronic circuit receives a high input voltage and therefore this form of voltage regulator is desirable. However, shunt regulators also tend to maintain the supply voltage at a level independent of the coil current and thus would remove any data carried by the relatively small amplitude modulation of the high frequency current. A shunt regulator that acts to control the mean supply voltage without at the same time suppressing the modulation component is therefore desired.
FIG. 1 shows a prior art shunt voltage regulator. Inductor coil L connects to a full wave rectifier that is made up of diodes D1, D2, D3 and D4. The inductor coil is tuned by a first capacitor C1. The output O of the full wave rectifier is connected to a load resistor R1 and a reservoir capacitor C2. The output O of the full wave rectifier is further connected to the drain of a NMOS transistor M that functions as a current sink. The gate of the NMOS transistor M is connected to the output of a voltage comparator COM through a low-pass filter LPF. The voltage comparator COM has an inverting input and a non-inverting input. The inverting input is connected to a reference voltage Vref while the non-inverting input is connected to the output O of the full wave amplifier. The comparator COM, the filter LPF and the MOS device M form a negative feedback loop that matches the rectifier output voltage to the reference voltage Vref. The filter LPF functions to prevent high frequency modulation, which carries data, from reaching the MOS device M so that the data is not removed from the output.
Although the shunt regulator shown in FIG. 1 provides adequate voltage protection to circuits in the smartcard, there are several disadvantages associated with this design. A first disadvantage of this circuit is that the transconductance of the MOS device M varies widely according to the current it is called upon to pass and hence the feedback loop characteristics can vary widely. A second disadvantage is that the rectifier circuit supplies current only during that phase of the energizing signal when the input voltage exceeds the sum of rectifier output voltage and the voltage drop across a pair of diodes. When the rectifier is not passing current, the MOS device M draws current from the reservoir capacitor C2, thereby producing a large ripple voltage on the output line. A third disadvantage is that because the MOS device M tends to act as a current sink, it presents a low dynamic conductance across the output. A consequence is that small variations in the received energy due to the modulation tend to produce exaggerated variations in the output voltage. A fourth disadvantage is that the transistor current returns to the coil via either diode D1 or D2. The coil terminal connected to the conducting diode of this pair will develop a voltage that is negative with respect the circuit's negative supply line by an amount equal to the diode voltage drop. This will tend to engender conduction in parasitic devices.
The prior art circuit show in FIG. 2 overcomes some of the disadvantages inherent in the FIG. 1 circuit by relocating the MOS device M directly across the coil. In this configuration, the MOS device M would not draw current from the reservoir capacitor C2 and so the circuit generates much less supply line ripple. Furthermore, the MOS current does not flow through the diodes and so negative excursions of the coil terminals with respect to the circuit's negative supply are avoided.
However, the disadvantage of having variable loop dynamics remains, and so does the tendency of the circuit to produce exaggerated modulation voltages due to the MOS device acting as a current sink. It would be desirable to have a voltage regulator circuit that can overcome the above-cited disadvantages.
An object of the present invention is to provide a voltage regulator circuit suitable for contactless smartcards that are inductively coupled to receive a high frequency energizing and data transmitting signal of variable power.
Another object of the invention is to provide a voltage regulator circuit that produces a regulated average supply voltage carrying an accurate image of amplitude modulated data contained in the high frequency input signal.